Synchronous linear electric motor and method for determining the commutation offset of a linear drive with such a synchronous linear electric motor

ABSTRACT

A synchronous linear electric motor with a secondary part that contains several permanent magnets arranged in a certain pole grid, and with a primary part that can move relative to the secondary part and whose position relative to the secondary part can be determined by a travel measurement system. For a simple determination of a commutation offset during startup of the linear motor, a mechanical marking is provided on the secondary part that has a known relationship to the pole grid of the permanent magnets. There is also a method for detecting the commutation.

FIELD AND BACKGROUND OF THE INVENTION

The invention relates initially to a synchronous linear electric motorwith a secondary part containing several permanent magnets arranged in acertain pole grid, and with a primary part that can move relative to thesecondary part and whose position relative to the secondary part can bedetermined by a travel measurement system.

The invention also relates to a method for determining the commutationoffset in a synchronous linear electric motor.

In order to ensure a force of the synchronous linear motor that isconstant over time, an angle of 90° must be set between the field vectorof the primary part and the field vector of the secondary part. Thismeans that the motor control of the primary part must operate as afunction of the current position of the location of the permanent magnetpoles of the secondary part. Therefore, the motor control requires thecurrent actual position values of the primary part relative to thesecondary part. A travel measurement system is provided for the purpose,which supplies this information for motor control.

In synchronous motors in general, following mechanical assembly, thereis initially no unambiguous relationship between the field vectors ofthe primary and secondary parts. This relationship is clearlyestablished during the initial startup of the drive by determining theso-called commutation offset.

SUMMARY OF THE INVENTION

It is an object of the invention to design a synchronous linear electricmotor in such fashion that it can be put in operation in simple fashionwith economical manufacture and reliable function. In addition, a highdegree of safety must also be ensured during assembly. A simple methodmust also be specified for startup.

A simple startup is possible with a synchronous linear electric motor inaccordance with the invention. In such a synchronous linear electricmotor, a mechanical marking is provided on the secondary part with aknown relationship to the pole grid of the permanent magnets. Such amarking can be used to determine the position of the primary partrelative to the permanent magnets of the secondary part, and hence thecommutation offset. After a relationship has been established betweenthe commutation offset and the position of the primary part at themoment as determined by the travel measurement system, the motor controlcan control the windings of the primary part such that an angle of 90°is set between the field vectors of the primary and secondary parts.

A method according to the invention is especially favorable fordetermining commutation offset, according to which claim the commutationoffset is calculated from the distance determined between the mechanicalmarking on the secondary part and the primary part as well as theposition of the marking to the pole grid internally in the motor controldevice.

Preferably, according to a feature of the invention, the relationship ofthe mechanical marking to the pole grid of the permanent magnets isstored in a nonvolatile manner in an electrical motor control device.

Advantageously, an absolute travel measurement system is used as thetravel measurement system. The commutation offset then does not have tobe determined again after each time the motor control device is switchedoff, but only when changes are made in the travel measurement system orwhen mechanical reconstruction with modified mounting conditions of theprimary or secondary parts has been performed.

Mounting bores are usually provided on the secondary part of thesynchronous linear electric motor, said bores serving to fasten thesecondary part to the machine bed. It is favorable to use a mountingbore of the secondary part directly or indirectly as a mechanicalmarking.

Several mechanical markings with the same position relative to the polegrid of the permanent magnets of the secondary part can also beprovided. The term "same position relative to the pole grid" means thatthe mechanical markings are located exactly in the middle of a northpole of the secondary part for example. The advantage of severalmarkings is that smaller distances have to be measured so that it may bepossible to avoid shifting the primary part. Basically, however, itmakes no difference which marking is used to determine the commutationoffset. In exactly the same way as when only one marking is present, adetermination is initially made, by external calculation orautomatically in the control device, as to how many whole-numbermultiples of the step width of the pole grid, i.e. the distance betweentwo like poles of the secondary part, are contained in the measureddistance. The remainder is critical for determining the commutationoffset.

The secondary part of a synchronous linear electric motor is usuallycomposed of several secondary individual parts with which travel lengthsof different sizes can be achieved depending on the number and length ofthe individual secondary parts. According to another feature of theinvention, each of the individual secondary parts is advantageouslyprovided with a mechanical marking with the same position relative tothe pole grid.

If the secondary part has a row of mounting bores that runs in thelengthwise direction, with all of the bores having the same positionrelative to the pole grid, all of these mounting bores can be used as amechanical marking. Preferably, the distance between the mounting borescorresponds to the step width of the pole grid. The mounting bores of asecond row that runs in the lengthwise direction of the secondary partadvantageously have different distances from one another. Therefore, thetwo rows can clearly be distinguished from one another, thusfacilitating the assembly of the secondary part from several individualsecondary parts. If all of the mounting bores in a row do not have thesame distance from one another, the assembly is not correct. The abilityto distinguish between the two rows of mounting bores also facilitatesthe correct arrangement of the primary part relative to the secondarypart. As an orientation aid on the primary part, its cable outlet isadvantageously mounted laterally.

Because of the difference in height between the secondary part and theprimary part, under certain conditions it is not as favorable for themechanical marking on the secondary part to be used to measure thespacing. It may be better, to use a reference device that can be locatedon the secondary part with a known relationship to the pole grid of thepermanent magnets, with device having a marking whose distance from theprimary part can be determined. Advantageously, an edge that runsperpendicular to the lengthwise direction of the secondary part on onelateral surface of the reference device can be used as the marking. Allof the available secondary parts are constructed identically to oneanother as far as the pole arrangement is concerned. The individuallengths of the individual secondary parts can be different. An importantfeature is that each north pole is followed by a south pole and viceversa in the lengthwise direction of the secondary parts.

BRIEF DESCRIPTION OF THE DRAWINGS

With the above and other objects and advantages in view, the presentinvention will become more clearly understood in connection with thedetailed description of a preferred embodiment, when considered with theaccompanying drawings of which:

FIG. 1 is a perspective view of an embodiment of a synchronous linearelectric motor according to the invention;

FIG. 2 is a top view of the embodiment, with the mechanical referencedevice mounted on the secondary part;

FIG. 3 is a cross section through the primary and secondary parts, withthe distance between the two parts being enlarged;

FIG. 4 shows in an enlarged scale, a top view of the mechanicalreference device itself in FIG. 2; and

FIG. 5 is a section along line V--V in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The arrangement of the synchronous linear electric motor in a machinecan be seen from FIG. 1. Secondary part 10, which is composed of severalsecondary individual parts 11 identical to one another as shown in FIG.2, is mounted on a machine bed 12 between two guide rails 13. Abridge-shaped machine carriage 14 is guided on these rails, saidcarriage supporting the primary part 15 at its underside that faces thesecondary part 10 of the linear motor. An absolute measuring rod 16 ofan absolute travel measurement system is mounted on the machine bed andextends in the lengthwise direction of guide rails 13 and of secondarypart 10 of the linear motor. The corresponding travel recorder 17 ismounted on machine carriage 14. The principle of the travel recorderalso applies in the reverse arrangement, i.e. the scale rod moves andthe scanning head is permanently attached to the part that remains atrest.

Primary part 15 comprises a block 20 in which electrical coils and linesare accommodated in a manner not shown in greater detail. A coolingplate 21 is mounted on the block. Cable 22 emerges laterally from an endface of the primary part 15 that points in the direction of thelengthwise extent of the linear motor and passes through a flexiblecable channel 23 to a motor control device 24.

All of the available secondary individual parts 11 are constructedidentically to one another relative to the magnetic pole arrangement andeach has permanent magnets 30. In the lengthwise direction of thesecondary part, a north pole always follows a south pole continuouslyand vice versa. The step width of the pole grid, i.e. the distancebetween two like poles, is equal to half the length of a secondaryindividual part 11. On both sides of permanent magnet 30, each secondaryindividual part 11 has two mounting bores 31 and 32, located in acarrier 33 for the permanent magnets 30. The distance between the twomounting bores 31 on one side corresponds to the step width of the polegrid. In addition, each of the two mounting bores 31 is located at adistance from the adjacent end of the secondary individual part that isequal to half the step width of the pole grid. Thus, when the secondaryindividual parts 11 are assembled so that all of the mounting bores 31are on the same side, a row of mounting bores is obtained, all of whichare located the same distance from one another. In addition, therelative positions of all the mounting bores 31 are the same relative tothe pole grid. Each mounting bore 31, as viewed in the lengthwisedirection of the secondary part 10, is located precisely between a southpole and a north pole located on the same side of the south pole in eachcase.

Mounting bores 32 on the other side of a secondary individual part 11are also each located at the same distance from the adjacent face. Thisdistance however is smaller than half the step width of a pole grid, sothat the distance between two mounting bores 32 of a secondaryindividual part 11 is greater than the step width of the pole grid.Therefore, on one side of secondary part 10 there is a row of mountingbores in which the distance between two mounting bores is alternatelylarger or smaller. The different arrangement of the mounting bores onthe two sides of a secondary individual part 11 thus facilitates thecorrect assembly of the secondary individual parts 11 to form a longsecondary part. On one side, there must be a row of mounting bores inwhich two successive mounting bores are located the same distance fromone another. On the other side, the spaces between successive mountingbores are alternately larger or smaller.

Primary part 15 is located relative to secondary part 10 in such fashionthat cable 22 emerges on the side of mounting bores 32.

The permanent magnets 30 are covered by a cover panel 34 that can beseen in FIG. 3 and which is secured to carrier 33 by the same bolts 35that are passed through mounting bores 31 and 32 and secure secondaryindividual parts 11 to the machine bed 12.

Each of the mounting bores 31 can be used as a mechanical marking withwhose aid the commutation offset of the primary part relative to thesecondary part can be determined during startup of the linear motor. Forthis purpose, the distance between the axis of a mounting bore 31 andthe end of the primary part 15 that has no cable outlet can bedetermined. This can be accomplished with the aid of a scale rod withoutfurther means. Even if measurement accuracy that is not very high isrequired for the interval measurement, because of the difference inheight between a mounting bore 31 and the primary part 15, it may bedifficult to determine the spacing sufficiently accurately. Therefore, areference device 40 is advantageously employed that can be secured in afixed position relative to bores 31 on the secondary part 10. Referencedevice 40 is an elongate profile made of aluminum for example, that hasa through stepped bore 41. This bore is aligned with a mounting bore 31for securing reference device 40 to secondary part 10, with no mountingbolt 35 in bore 31 or with mounting bolt 35 having been unscrewed. Thena bolt 35 is passed through stepped bore 41 and mounting bore 31 and isscrewed into a threaded bore in the machine bed. Reference device 40extends up to the axis of an adjacent mounting bore 31 that is locatedin an end 42 of the reference device 40 that extends perpendicularly tothe lengthwise direction of secondary part 10. The head of a bolt 35located in the adjacent mounting bore can be received In a recess 43 atend 42. The distance between end 42 and primary part 10 corresponds tothe distance from the corresponding mounting bore 31. In order todetermine the commutation offset when the synchronous linear motor isstarted, the distance between end 42 of the reference device 40 and thefacing end of the primary part 15 is measured. During measurement, theprimary part must reliably maintain its position relative to thesecondary part until the next input of the measured values. The measureddistance is entered into the motor control device, which calculates thecommutation offset from the stored position of the mounting bores 31relative to the permanent magnets 30. Only a remaining distance is takeninto account in the course of this process that is obtained if the stepwidth of the pole grid is subtracted as often as possible in advance. Inaddition to secondary part 10, the position of primary part 15 relativeto the travel measurement system is also taken into account. The motorcontrol now knows how primary part 15 is to be controlled when it is ina certain position as detected by the travel measurement system.

We claim:
 1. Synchronous linear electric motor with a secondary part(10) that has several permanent magnets (30) arranged in a certain polegrid, and with a primary part (15) that is movable relative to thesecondary part (10), and wherein the position of the primary partrelative to the secondary part (10) is detectable by a travelmeasurement system (16, 17), and a mechanical marking (31) is located onthe secondary part (10) with a known relationship to the pole grid ofthe permanent magnet, the pole grid serving as a part of a travelmeasurement system;wherein, during operation of the motor, themechanical marking serves to determine the relative initial position ofthe primary part with respect to the pole grid; and after associatingthe initial position of the primary part relative to the pole grid, themechanical marking enables an electric motor control unit to drive theprimary part as a function of a displacement detected by the travelmeasurement system.
 2. Synchronous linear electric motor according toclaim 1, further comprising said electrical motor control device (24) inwhich the relationship of a mechanical marking (31) to the pole grid ofthe permanent magnets (30) is stored in a nonvolatile manner. 3.Synchronous linear electric motor according to claim 1, comprising saidabsolute travel measurement system (16, 17) for detecting the positionof the primary part (15) relative to the secondary part (10). 4.Synchronous linear electric motor according to claim 1, wherein amounting bore (31) in the secondary part (10) serves directly orindirectly as the mechanical marking.
 5. Synchronous linear electricmotor according to claim 1, wherein several mechanical markings (31) inaddition to said mechanical marking are provided that have the samepositions relative to the pole grid of the permanent magnets (30) of thesecondary part (10).
 6. Synchronous linear electric motor according toclaim 1, wherein the secondary part (10) is composed of severalpreferably identical secondary individual parts (11) each of which has amechanical marking (31) with the same position relative to the pole gridof the permanent magnets (30).
 7. Synchronous linear electric motoraccording to claim 4, comprising a row of mounting bores (31) on thesecondary part (10) that extend in lengthwise direction of the secondarypart (10), said bores all having the same position relative to the polegrid.
 8. Synchronous linear electric motor according to claim 7, furthercomprising a second row of mounting bores (32) running in the lengthwisedirection of the secondary part (10), said bores (32) being spaced atdifferent distances from one another.
 9. Synchronous linear electricmotor according to claim 8, further comprising a lateral outlet for acable (22) on the primary part.
 10. Synchronous linear electric motoraccording to claim 1, further comprising a mechanical reference device(40) locatable in a known relationship to the pole grid of the permanentmagnets (30), said device having a marking (42) whose distance from theprimary part (15) can be determined.
 11. Synchronous linear electricmotor according to claim 10, wherein an edge of a lateral surface (42)of the reference device (40) that extends perpendicularly to thelengthwise direction of the secondary part (10) serves a marking. 12.Synchronous linear electric motor according to claim 10, wherein thereference device (40) can be placed in a fixed relationship to themounting bores (31) of the secondary part (10) by using at least onemounting bore (31) of the secondary part (10).
 13. Method fordetermining the commutation offset in a linear drive with a synchronouslinear electric motor with features of claim 2, comprising the steps ofdetermining the distance between the mechanical marking (31) placed onthe secondary part (10) and a predefined point on the primary part (15),entering or automatically reading the distance thus determined into amotor control device (24), and calculating the commutation offset in themotor control device (24) from the position of the marking (31) relativeto the pole grid and the distance of the marking (31) from thepredefined point on the primary part (15).
 14. Synchronous linearelectric motor according to claim 2, comprising said absolute travelmeasurement system (16, 17) for detecting the position of the primarypart (15) relative to the secondary part (10).
 15. Synchronous linearelectric motor according to claim 4, wherein several mechanical markings(31) in addition to said mechanical marking are provided that have thesame positions relative to the pole grid of the permanent magnets (30)of the secondary part (10).
 16. Synchronous linear electric motoraccording to claim 7, the spacing of the mounting bores from one anothercorresponds to the step width of the pole grid.
 17. Synchronous linearelectric motor according to claim 16, further comprising a second row ofmounting bores (32) running in the lengthwise direction of the secondarypart (10), said bores (32) being spaced at different distances from oneanother.
 18. Synchronous linear electric motor according to claim 17,further comprising a lateral outlet for a cable (22) on the primarypart.
 19. Synchronous linear electric motor according to claim 11,wherein the reference device (40) can be placed in a fixed relationshipto the mounting bores (31) of the secondary part (10) by using at leastone mounting bore (31) of the secondary part (10).